>but calling the system's built-in application installation mechanism "not primary" is absurd.
So you're arguing that because play store installs and random .apk installs both goes through packageinstaller, the concept of a "primary" install method doesn't exist?
If we're using "primary" to mean "first-party" (as in your original comment), then the system's built-in package installer is the most first-party of all, so it's definitely not "not primary".
If we're using "primary" to mean something like "most popular", then I don't see how the term "sideloading" would make any sense to describe "not primary". Are we side-commenting here, and side-submitting HTTP requests, because we're not posting to Facebook, the primary website?